1. Field of the Invention
The invention pertains to a transducer of time-related temperature variations into a difference in potentials. The invention also pertains to an electronic chip for detecting a heat pattern incorporating this transducer as well as to a method for fabricating this chip.
Typically, an electronic chip is made by a method of microelectronic fabrication, i.e. by the deposition of successive layers on the substrate and by the etching of certain of these layers, for example by photolithography.
2. Description of the Prior Art
Prior-art transducers comprise:                an upper conductive electrode designed to be exposed to the time-related temperature variation to be measured,        a lower conductive electrode,        at least one layer of pyroelectric material directly interposed between these electrodes to generate, between these electrodes, the difference in potentials corresponding to the temperature variation even when there is no external mechanical stress.        
For example, a transducer of this kind is disclosed in the U.S. Pat. No. 4,429,413 in the context of a fingerprint detector.
An external mechanical stress is a mechanical stress applied to the upper electrode by a movable object external to the transducer.
It is important that the pyroelectric material used should have a pyroelectric coefficient that is as high as possible. The term “pyroelectric coefficient” herein designates the ratio defined by the following relationship: γ=ΔP/ΔT, where:                ΔT is the temperature variation, and        ΔP is the difference in potentials obtained in response to the temperature variation ΔT.        
The coefficient γ must be high because the temperature variations to be detected are very low, i.e. below 1° C. and preferably 0.1° C. Furthermore, the heat exchange surface area between the transducer and the object applied to this transducer is often very small, i.e. less than 1 mm2 and typically less than 5000 or 3000 μm2. It is therefore necessary to use a pyroelectric material that can give the greatest possible difference in potentials in response to these low temperature variations in order to amplify the signal.
Prior-art transducers are therefore made uniquely with pyroelectric materials that have a high pyroelectric coefficient, i.e. a coefficient greater 20 μC/m2/K. For example, the pyroelectric materials used are:                a) polymers such as polyvinyldifluoride (PVDF) for which the pyroelectric coefficient is of the order of 40 μC/m2/k,        b) ceramics such as the ceramic known by the acronym LZT (lead zirconate titanate) for which the pyroelectric coefficient is about 350 μC/m2/K, or        c) crystals such as those known by the acronym TOTS (Triglycine SDF) or LiTaO3 crystals.        
It is also desirable that the size of the transducer should be as small as possible to increase its spatial resolution. For example, the sensitive face of the transducer should be smaller than 1 mm2 and preferably smaller than 0.5 or 0.3 mm2.
The sensitive face of the transducer is the face of this transducer through which the temperature variation to be converted into a difference in potential is picked up.
Because of its small dimensions, the transducer must preferably be fabricated by methods of microelectronic fabrication, i.e. the same collective fabrication methods as those used to fabricate the electronic chips used to make computers or the like. For example, these collective fabrication methods use the deposition and etching of layers stacked one upon the other on a substrate. Typically, the substrate is a wafer made of silicon or glass.
The prior art pyroelectric materials having a high pyroelectric coefficient are difficult to use in microelectronic fabrication methods.
For example, the polymer PVDF must be polarized after deposition with a voltage of some hundreds of volts and once polarized the other fabrication operations must be performed at temperatures inferior to 120° C. so that this polymer PVDF does not lose its pyroelectric properties.
Ceramics require high fabrication temperatures, i.e. temperatures of over 600° C. and therefore far higher than a temperature of 400 to 450° C. generally given as the upper limit for implementing a method of microelectronic fabrication. Thus, the prior-art transducers are difficult to fabricate